That's the error I am getting all too frequently. It happens when I edit a numeric value. Changing font size, changing color via HSB or RGB values, etc. Sometimes I can hit OK on the error message a few times and I can keep working. Other times, I hit OK but it keeps coming back and I have to Force Quit. I have had to Force Quit 4 times so far today.

What version of After Effects? Have you installed the recent updates? What kind(s) of source footage? Has this problem always existed? If not, what has changed? What other software are you running? Do you have any third-party effects or codecs installed? Tell us about your computer hardware. Are you using OpenGL?